Add .editorconfig to make it easier to obey style

Changelog-None

.editorconfig

@@ -0,0 +1,20 @@
+# https://editorconfig.org/#file-format-details
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[{**.c,**.h,Makefile}]
+indent_style = tab
+
+[**.py]
+indent_style = space
+indent_size = 4
+
+[.travis.yml]
+indent_style = space
+indent_size = 2